Reference Implementation Update

[deanstef]

This update to the reference implementation introduces the following changes:

• Authentication message: introduced chainId parameter that specifies the Algorand network in which the authentication account operates;
• Authentication request: the client specifies the chainId for the authentication account
• Simplified authentication message encoding
  • removed unused prefix arc31:j
  • added arc0031 prefix compliant with Algorand domain sepration
  • removed msgpack in favour of pure JSON (rational: auth message is not a large object and does not require particular msgpacking)
• Added chainId entry to user mock DB: the Verifier must keep track of the chainId to use for signature validation